WORKS IN PROGRESS

WORKS IN PROGRESS; Before the Dinosaurs

By BRUCE WEBER

Published: August 23, 1987

''PLACERIAS LOOKED LIKE a reptilian rhinoceros,'' says Robert A. Long, a research associate at the Museum of Paleontology at the University of California at Berkeley. ''It averaged about 10 feet long and weighed 2 or 3 tons. It had a huge head with a turtlelike beak, a pair of tusks and a short tail. It walked on all four legs and its feet were massive.'' Placerias is one of the last dicynodonts, a reptile group related to the ancestors of mammals but that itself was an evolutionary dead end. This fall, Long's reconstruction of a Placerias skeleton, now being completed, will be on exhibit, mounted and standing erect, at the Rainbow Forest Museum in Arizona. It will be the first public appearance for Placerias in about 225 million years.

The museum is in Petrified Forest National Park and is near the site where, in 1930, 1,600 Placerias bones were discovered - along with thousands of other bones once belonging to a variety of other reptiles, including early, pet-sized dinosaurs. Together, they constitute the largest variety of fossil reptile bones of that age yet discovered on earth.

Photographed above in their Berkeley laboratory, Long and Roderick T. McCrea (seated), a sculptor and scientific illustrator, incorporated the bones of at least a dozen separate animals in the completed skeleton, more than 300 bones in all. ''We've reconstructed all the broken and missing bones - we were missing a number of ribs,'' Long says. Temporary facsimiles were made of clay; before mounting, they'll be replaced with bones of fiberglass.

Placerias is one of several reconstructions Long is planning, all of them creatures fossilized at the Arizona site. ''They were all reptiles, all land-living. Most have not received popular attention.''

Photos of dinosaur skeleton; paleontologist Robert Long (Paul Fusco/Magnum)